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The galvanomagnetic characteristics of SmB6 single crystals are studied within the temperature range of 1.9‒3.6 K at different orientations of the crystal faces. As a result, the electrical resistivities of the surfaces corresponding to the (100), (110), (111), and (211) crystallographic planes are determined. It is shown that the effective parameters of charge carriers, which determine the surface conductivity in SmB6, depend both on the orientation of the surface and on the method of its processing. It is found that the etching of polished polar surfaces formed by (100) planes leads at 1.9 K to a decrease in the density and to an increase in the mobility of surface n-type charge carriers from 113/a2 and 1.12 cm2/(V s) to 0.76/a2 and 18 cm2/(V s), respectively (lattice parameter a ≈ 4.134 Å). For etched nonpolar surfaces corresponding to the (110) and (111) planes, the maximum density of surface charge carriers (per unit area of the surface Brillouin zone) is found to increase by factors of 2.3 and 3.9, respectively. It is proposed to use this parameter as a simple criterion to identify the features of electron transport due to the nontrivial topology of the band structure of SmB6. [ABSTRACT FROM AUTHOR]

Detailed measurements of the Hall effect in the paramagnetic phase of Ho0.8Lu0.2B12 antiferromagnet at the magnetic field up to 80 kOe in the temperature range of 1.9–300 K have been performed. It has been found that the transition to the cage glass phase (T < T * ~ 60 K) is accompanied by the appearance of a positive Hall resistance component in addition to that corresponding to the negative Hall effect. The amplitude and angular dependence of the former depend on the magnitude and direction of the applied magnetic field with respect to the crystallographic axes. The revealed anisotropy of the Hall effect in Ho0.8Lu0.2B12 is attributed to the interaction of charge carriers with dynamic charge stripes. [ABSTRACT FROM AUTHOR]

The effect of symmetry lowering for the parameters of electron transport in rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Lu) with fcc crystal structure at temperatures T ≤ 150 K has been discovered. It has been shown that the anisotropy of the magnetoresistance increases sharply at the transition to the disordered cage-glass phase below T* ~ 60 K. This effect is observed in magnetic and nonmagnetic rare-earth dodecaborides, including solid solutions RxLu1–xB12 with high substitutional disorder. The role of the electron phase separation (charge stripes along the 〈110〉 direction) in these metals with open trajectories on the Fermi surface is discussed. [ABSTRACT FROM AUTHOR]

The temperature dependence of the conductivity of the topological Kondo-insulator (TKI) Sm1 –xYbxB6 is investigated in the temperature range 2 < T < 300 K for compositions with x ≤ 0.024. It is found that the ytterbium impurity most strongly affects the low-temperature (T < 20 K) electrical conductivity: when changing from x = 0 to x = 0.024, the activation energy of bulk conductivity decreases by 1.8 times from 4 to 2.2 meV, and the 2D surface conductivity increases by six times. The separation made for the contributions to the conductivity enables us to establish that the one-parameter scaling model can be used for describing the conductivity of 2D surface states in the Sm1 –xYbxB6 TKI in which both the interaction with phonons and the electron–electron scattering effects are taken into account. [ABSTRACT FROM AUTHOR]

The low temperature (T < 5 K) electron spin resonance in topological Kondo insulator SmB6 is investigated. It is shown that spin relaxation time of the paramagnetic centers responsible for low temperature electron spin resonance in SmB6 is about ~ 10−8 s, which exceeds by five orders of magnitude the widely accepted spin and charge fluctuation time ~ 10−13 s in the homogeneous mixed valence state of this material. This discrepancy may be attributed to the formation of the specific paramagnetic centers in the sample surface–spin polarons. [ABSTRACT FROM AUTHOR]

The transverse magnetoresistance, heat capacity, and magnetization of monodomain single crystals of dodecaboride Ho0.5Lu0.5B12 with a cage-glass structure are investigated at helium temperatures. It is shown that the anisotropy of charge carrier scattering in the antiferromagnetic phase is due to dynamic charge stripes oriented along directions 〈110〉 in the fcc lattice. [ABSTRACT FROM AUTHOR]

The magnetic properties of the topological Kondo insulator SmB6 are studied in the temperature range T < 10 K in an applied magnetic field up to 5 T. The analysis of the nonlinear part of the field dependence of the magnetization indicates that the nonlinear contribution to the magnetization at low temperatures is in general related not only to localized magnetic moments but also to the Pauli paramagnetism of surface electrons. It has been shown for the first time that a decrease in temperature in the range of T* ∼ 5.5–6 K gives rise to the crossover between two modes determining the magnetic properties of SmB6. In the range T > T*, the magnetization qualitatively corresponds to the model of decay of Kondo singlets and is determined by the Pauli paramagnetism, which includes both the linear and nonlinear contributions. In the range T < T*, the Pauli paramagnetism becomes supplemented in the threshold manner by the contribution of localized magnetic moments. The performed estimate of effective magnetic moments arising at T < T* gives anomalously high values μ* ∼ (7–14) μB ion, far exceeding the values μ* ∼ (3–5) μB expected for a separate magnetic Sm3+ ion. This is a signature of the possible spin-polaron nature of localized magnetic moments in the topological Kondo insulator SmB6. [ABSTRACT FROM AUTHOR]

A study of the charge transport, magnetization, and high frequency (ω/2π=60 GHz) electron spin resonance (ESR) has been carried out on La1-xCaxMnO3 single crystals corresponding to the dielectric side of the concentration driven metal-insulator transition (x≤xMIT∼0.22). For x=0.22 the onset of ferromagnetism below the Curie temperature TC≈183 K was found to induce crossover from holelike (T>TC) to electronlike (TC) conductivity. The activation behavior of resistivity and Seebeck coefficients with the respective activation energies Eaρ∼123 meV and EaS∼32 meV was detected in the paramagnetic state of x=0.22 compound for T>TC. A large enhancement of the Hall effect, which is accompanied by the increase in Hall mobility to the values of μH∼-20 cm2/V s, was observed at intermediate temperatures 150 KC. Besides, a single ESR line with g factor g≈2 observed for x=0.2 at T>TC was shown to evolve into an intricate two-mode ESR spectrum with irregular fine structure characterizing the ferromagnetic insulating (FI) state formed below TC. The anomalies of charge transport and magnetic properties found in FI state seem to be associated with the peculiarities of strongly disordered state with phase separation (ferromagnetic metallic domains embedded in the insulating matrix of La1-xCaxMnO3). [ABSTRACT FROM AUTHOR]

The specific heat of Ho0.01Lu0.99B12 single crystals with dynamic charge stripes is studied at low and ultralow temperatures (0.07-20 K) in applied magnetic fields of 0-9 Т. It is shown that additional contributions to the specific heat appear at magnetic fields exceeding 1 T. These contributions depend on the orientation of the magnetic field and can be as large as 15% of the specific heat related to the subsystem of holmium ions in the cubic crystal field. [ABSTRACT FROM AUTHOR]

The transverse magnetoresistance of Ho0.8Lu0.2B12 dodecaboride with a cage glass structure is studied at low (2210 K) temperatures. It is demonstrated that the isotropic negative magnetoresistance in this antiferromagnet is dominant within the broad temperature range near TN K. This contribution to the total magnetoresistance is due to the scattering of charge carriers by nanoclusters formed by 3+ ions, and it can be scaled in the = f((²effH²/T²) representation. It is found that the magnetoresistance anisotropy above (about 15% at 80 kOe) is due to the positive contribution, which achieves maximum values at the magnetic field direction close to H [001]. The anisotropy of the charge carrier scattering is interpreted in terms of the cooperative dynamic JahnnTeller effect at B 12 clusters. [ABSTRACT FROM AUTHOR]

Magnetoresistance (H,T) of several heavy-fermion compounds, CeAl2, CeAl3 and CeCu6, substitutional solid solutions with quantum critical behavior CeCu6-xAux (x = 0.1, 0.2) and alloys with magnetic ground state Ce(Al1-xMx)2 (M = Co, Ni,x = 0.8) was studied in a wide range of temperatures (1.8-40 K) in magnetic fields up to 80 kOe. It was shown that a consistent interpretation of the field dependences of the resistance for both non-magnetic and magnetically ordered cerium-based intermetallic compounds with strong electron correlations can be achieved within the framework of an approach that accounts for scattering of charge carriers by localized magnetic moments in a metal matrix. Within this approach, three different components of the magnetoresistance of ceriumintermetallic compounds were identified: the negative Brillouin contribution proportional to the local magnetization (-/~M2loc-/~Mloc2), the alternating linear contribution (/~H/~H) and the magnetic component, saturating in magnetic fields below 15 kOe. In the framework of the Yosida model for the cerium alloys under study, estimates of the local magnetic susceptibility loc(H, T0) were obtained from the magnetoresistance data. Numerical differentiation of the magnetoresistance with respect to the magnetic field and analysis of the obtained d/dH=f(H,T)d/dH=f(H,T) dependences allowed us to reconstruct the H-T magnetic phase diagrams of the strongly correlated electron systems under study as well as to examine the effects of spin polarization and renormalization of the electronic states on charge transport both in the regime of quantum critical behavior and in the magnetically ordered state. [ABSTRACT FROM AUTHOR]

High frequency (60 GHz) electron spin resonance (ESR) and magnetic susceptibility were studied for VOx multi-walled nanotubes (VOx-NTs) in the range of 4.2-300 K. It is found that apart from technological impurities, the magnetism of VOx-NTs is controlled by (i) antiferromagnetic V4+ noninteracting dimmers and (ii) V4+ quasi-free spins, which at T < 50 K form a disorder driven quantum critical state. From concentration estimates, we argue the V4+ dimers are an essential element of the VOx-NTs structure. [ABSTRACT FROM AUTHOR]

The transport and magnetic properties of single crystal samples of substitutional solid solutions EuCaB (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8-300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) − ρ( H))/ρ( H) ≈ 7 × 10 detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the EuCaB solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200−350 cm/(V s) estimated for charge carriers in the strongly disordered matrix of the EuCaB solid solution are comparable with the charge carrier mobility μH = 400−600 cm/(V s) for the undoped EuB compound. The anomalous behavior of the transport and magnetic parameters of the EuCaB solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density. [ABSTRACT FROM AUTHOR]

The galvanomagnetic properties of single-crystal samples with various isotopic boron compositions have been investigated for the first time for the normal state of superconductor LuB ( T c ≈ 0.44 K). Precision measurements of the resistivity, Hall coefficient, and magnetic susceptibility have been performed over a wide temperature range of 2-300 K in magnetic fields up to 80 kOe. A change of the charge transport regime in this nonmagnetic compound with metallic conduction is shown to occur near T* ≈ 50−70 K. As a result, a sharp peak with significantly different amplitudes for LuB and LuB is recorded in the temperature dependences of the Hall coefficient R( T) near T*. A significant (about 10%) difference (in absolute value) of the Hall coefficients R for the LuB and LuB compounds at helium and intermediate temperatures has been found and the patterns of behavior of the dependence R( H) for T < T* in an external magnetic field H ≤ 80 kOe for LuB and LuB are shown to differ significantly. Analysis of the Curie-Weiss contribution to the magnetic susceptibility χ( T) leads to the conclusion about the formation of magnetic moments μ ≈ (0.13−0.19)μ in each unit cell of the fcc structure of LuB compounds with various isotopic compositions. The possibility of the realization of an electronic topological 2.5-order transition near T* and the influence of correlation effects in the 5 d-band on the formation of a spin polarization near the rare-earth ions in LuB is discussed. [ABSTRACT FROM AUTHOR]

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm1 − xYb xB12 system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm0.74Yb0.26B12 samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/ T ∼ ln T at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm0.74Yb0.26B12 has been analyzed. [ABSTRACT FROM AUTHOR]

The magnetic properties of nanomaterials based on vanadium oxide (multiwall nanotubes, nanorods, and nanolayers) have been investigated in the temperature range of 1.8–220 K by high-frequency (60-GHz) EPR. A transition from a ferromagnetic temperature dependence to an antiferromagnetic temperature dependence has been observed in nanorods and nanotubes with a decrease in the temperature. The FM-AFM crossover observed near TC ∼ 110 K is accompanied by a low-temperature increase in the Curie constant by a factor of 2.7–7. The comparison of the experimental data for various VO x nanoparticles indicates that the most probable cause of the change in the type of magnetic interaction is a change in the concentration of V4+ magnetic ions. [ABSTRACT FROM AUTHOR]

The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB6 and NdB6) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB6 and NdB6 compounds. An analysis of the dependences Δρ( H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H2), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB6 and NdB6 compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB6 and NdB6, ferromagnetic nanoregions (ferrons) are formed in the 5 d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χloc has been estimated. It has been demonstrated that, in the temperature range TN < T < 20 K, the behavior of the local magnetic susceptibility χloc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χloc ∝ ( T − Θ p)−1. [ABSTRACT FROM AUTHOR]

In isotopically pure praseodymium hexaboride (Pr11B6) single-crystal samples, the transverse magnetoresistance Δρ/ρ has been measured in a temperature range of 2–20 K in magnetic fields up to 80 kOe. The field and angular dependences Δρ/ρ( H, ϕ, T0) reveal a new magnetic phase in the AFM state of Pr11B6 which is observable only for the external magnetic field orientation in a narrow angular range near H | 〈110〉. The data remove the previous contradictions in the Pr11B6 magnetic phase diagram representation and can be explained under the assumption that the spin-polarized regions (ferrons) are involved in the formation of the complex magnetic structure in the Pr11B6 AFM state in the 5 d band in the vicinity of the rare-earth ions. [ABSTRACT FROM AUTHOR]

The results of Hall-effect and resistance measurements on the substitutional solid solutions CeCu6-xAux with concentrations 0≼x≼0.3, corresponding to a wide neighborhood of the quantum critical point (QCP) at x=0.1, are presented. The measurements are made by rotation of the sample in a constant magnetic field of up to 70 kOe in a temperature interval of 1.8–300 K. For the classic heavy-fermion compound CeCu6 the temperature dependence of the Hall coefficient RH(T) exhibits a complex activational form with activation energies Ea1/kB≈110 K and Ea2/kB≈1.5 K in the temperature intervals 50–300 K and 3–10 K, respectively. It is shown that the anomalous behavior of the Hall effect can be explained in a spin-polaron approach, in which the values Ea1,2 can be associated to the binding energy of many-body and one can obtain estimates of the effective mass (meff1,2≈130–150m0) and localization radius (ap1,2*≈1.7 and 14 Å) of the charge carriers in CeCu6. For the compound CeCu5.9Au0.1, corresponding to the QCP, one observes correlated power-law behavior of the temperature dependence of the Hall coefficient RH(T)∼T-0.4 and magnetic susceptibility χ(T)∼T-0.4, as is characteristic of the regime of quantum critical behavior. For compounds in the immediate vicinity of the QCP an anomalous, even contribution RH2 to the angular dependence of the Hall voltage appears at temperatures below T*∼24 K and becomes stronger with increasing magnetic field. Different scenarios for passage through the QCP and their applicability for describing the Hall-effect anomalies in the substitutional solid solutions CeCu6-xAux are discussed. [ABSTRACT FROM AUTHOR]

The magnetic and transport characteristics of substitutional solid solutions of the Tm1− xYb xB12 rare earth dodecaborides have been investigated. The measurements performed in the wide temperature range 1.8−300 K on the high-quality single-crystalline samples make it possible to conclude that as x increases, antiferromagnetic instability develops with the quantum critical point ( T N = 0) near x = 0.3 and relevant dielectrization of the electron structure in the range 0 ≤ x ≤ 0.8 takes place. With the results of the thermoelectric measurements in Tm1− xYb xB12, the activation energy values and their dependence on x have been determined. [ABSTRACT FROM AUTHOR]

The magnetoresistance and magnetization of single-crystal samples of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Lu) have been measured at low temperatures (1.8–35 K) in a magnetic field of up to 70 kOe. The effect of positive magnetoresistance that obeys the Kohler’s rule Δρ/ρ = f(ρ(0, 300 K) H/ρ(0, T)) is observed for the nonmagnetic metal LuB12. In the magnetic dodecaborides HoB12, ErB12, and TmB12, three characteristic regimes of the magnetoresistance behavior have been revealed: the positive magnetoresistance effect similar to the case of LuB12 is observed at T > 25 K; in the range T N ≤ T ≤ 15 K, the magnetoresistance becomes negative and depends quadratically on the external magnetic field; and, finally, upon the transition to the antiferromagnetic phase ( T < T N), the positive magnetoresistance is again observed and its amplitude reaches 150% for HoB12. It has been shown that the observed anomalies of negative magnetoresistance in the paramagnetic phase can be explained within the Yosida model of conduction electron scattering by localized magnetic moments. The performed analysis confirms the formation of spin-polaron states in the 5 d band in the vicinity of rare-earth ions in paramagnetic and magnetically ordered phases of RB12 and makes it possible to reveal a number of specific features in the transformation of the magnetic structure of the compounds under investigation. [ABSTRACT FROM AUTHOR]

The angular dependences of the magnetization and Hall resistance have been investigated by the method of the sample rotation in the magnetic field in the high-quality single-crystal samples in the paramagnetic and magnetically ordered phases of CeB6 in the magnetic field up to 60 kOe. It has been shown that, as CeB6 undergoes the transition from the antiferromagnetic modulated phase to the so-called antiferroquadrupolar phase, the easy-magnetization axis in the [110] plane changes from 〈100〉 to 〈110〉. The magnetic field dependences of the anisotropic component of the magnetization differ radically in these magnetically ordered phases. The analysis provides evidence in favor of the formation of a state with the spin density wave (SDW phase) in the temperature range TN ≈ 2.3 K < T < TQ ≈ 3.3 K in CeB6. [ABSTRACT FROM AUTHOR]

Procedure of the hopping conductivity ρ∼exp[(T 0/T)α] and magnetoresistance data analysis, which takes into account spin-polarization contribution and allows finding correct parameters of the localized states, is suggested. This general approach was checked for the nanocluster material synthesized under high (up to 8 GPa) pressure of carbon SWNT mat containing 5% Ni. Experiments for the temperature/field domain 1.8-300 K/70 kOe have confirmed theoretical results, including universal magnetoresistance scaling ln[ρ(H)/ρ(0)] = (T 0/T)α F(H/T). It is found that the interaction with magnetic impurities renormalizes the electron magnetic moment: μ≈0.7μB. In conclusion, a possible way of creation of the carbon nanocluster material with colossal magnetoresistance is suggested. [ABSTRACT FROM AUTHOR]

Within a wide temperature range of 1.8–300 K, the Hall effect in holmium dodecaboride is investigated. The measurements of the angular dependences of the Hall resistance ρH(φ, T, H) for HoB12 performed in a high magnetic field up to 80 kOe at helium and intermediate temperatures made it possible to separate the normal and anomalous contributions to the Hall effect. Analysis of the anomalous component behavior in the paramagnetic and Néel phases suggests the appearance of the 5 d-state magnetic polarization (the spin polaron effect) and makes it possible to compare the revealed features of the Hall coefficient RH( T, H) with the H-T magnetic phase diagram of HoB12. [ABSTRACT FROM AUTHOR]

The influence of substitution on the binding energy of many-body states and the formation of the magnetically ordered state in a heavy-fermion compound (CeAl2) have been studied by measuring the transport characteristics (Hall effect, resistivity) in intermetallic compounds of the Ce(Al1− x M x )2 system (M = Ni, Co; x ≤ 0.08). It is established that the Hall coefficient R H in Ce(Al1− x Co x )2 intermetallides with x = 0.05 and 0.08 grows by more than an order of magnitude as the temperature decreases from 1.8 to 300 K. The experimental data are used to estimate the effective mass of charge carriers, the relaxation time, and the localization radius of many-body states. [ABSTRACT FROM AUTHOR]

High-precision measurements of thermopower have been performed in a wide temperature range (2–300 K) for a series of cerium-based heavy-fermion compounds, including CeB6, CeAl3, CeCu6, and substitutional solid solutions of the CeCu6 − x Au x system ( x = 0.1, 0.2). All compounds exhibit an unusual (logarithmic) asymptotic behavior of the temperature dependence of the Seebeck coefficient: S ∝ −ln T. In the case of cerium hexaboride, this anomalous behavior of S( T) is accompanied by the appearance of weak-carrier-localization-mode asymptotics in the conductivity (σ( T) ∝ T 0.39), while the paramagnetic susceptibility χ( T) and the effective mass of charge carriers m eff( T) vary according to a power law (χ( T), m eff( T) ∝ T −0.8) in the temperature interval T = 10–80 K. This behavior corresponds to renormalization of the density of states at the Fermi level. The observed anomalous behavior of thermopower in CeB6 and other cerium-based intermetallic compounds is attributed to the formation of heavy fermions (many-body states in the metal matrix) at low temperatures. [ABSTRACT FROM AUTHOR]

Precision measurements of transport and magnetic parameters of high-quality CeB6 single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T −1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ( T) ∝ T −0.8 is obtained for magnetic susceptibility of CeB6 in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB6 and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB6 is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB6 matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at T Q ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB6, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions. [ABSTRACT FROM AUTHOR]

The Hall coefficient RH , resistivity ρ, and Seebeck coefficient S of the CeAl2 compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences RH(φ, T, H ≤ 70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic RHam and main RHa components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component RHam were used to analyze the complex magnetic phase diagram H–T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution RHa(H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl2 intermetallic compound. The results led us to conclude that taking into account spin–polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl2 matrix were estimated to be (55–90)m0 and 6–10 Å, respectively. The behaviors of the parameters RH, S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl2 . © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The Hall coefficient R[sub H] and magnetoresistance of a magnetic Kondo lattice of CeAl[sub 2] were investigated over a wide temperature range from 1.8 to 300 K in magnetic fields of up to 80 kOe. Analysis of the measured angular dependences R[sub H](φ,T,H) made it possible to separate the contributions of skew scattering and anomalous magnetic scattering to the anomalous Hall effect. The results obtained were compared with the existing theoretical models. © 2002 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The conductivity, thermopower, and magnetoresistance of carbynes structurally modified by heating under a high pressure are investigated in the temperature range 1.8-300 K in a magnetic field up to 70 kOe. It is shown that an increase in the synthesis temperature under pressure leads to a transition from 1D hopping conductivity to 2D and then to 3D hopping conductivity. An analysis of transport data at T ≤ 40 K makes it possible to determine the localization radius a ∼ (56-140) Å of the wave function and to estimate the density of localized states g(E[sub F]) for various dimensions d of space: g(E[sub F]) ≈ 5.8 × 10[sup 7] eV[sup -1] cm[sup -1] (d=1), g(E[sub F]) ≈ 5 × 10[sup 14] eV[sup -1] cm[sup -2] (d= 2), and g(E[sub F]) ≈ 1.1 × 10[sup 21] eV[sup -1] cm[sup -3] (d = 3). A model for hopping conductivity and structure of carbynes is proposed on the basis of clusterization of sp² bonds in the carbyne matrix on the nanometer scale. [ABSTRACT FROM AUTHOR]

Low-temperature anomalies in the physical properties of iron monosilicide are analyzed based on the results of thorough measurements of the conductivity, Hall coefficients, thermo emf, and magnetic characteristics of high-quality single-crystal FeSi samples at liquid helium (LHe) and intermediate temperatures. It is demonstrated that the most adequate and consistent interpretation of the experimental magnetic, transport, and optical characteristics can be given within the framework of the Hubbard model. The model parameters are determined and the arguments are presented which provide evidence of the spin polaron formation and the density of state (DOS) renormalization taking place in FeSi in the vicinity of the Fermi energy at intermediate temperatures. It was found that a decrease in the sample temperature in the region of T < T[sub c]approx. 15 K is accompanied by a transition to a coherent regime of the spin density fluctuations. As a result, the ferromagnetic character of the interaction leads to the formation of magnetic microdomains with a characteristic size ∼10 Å. The exchange-induced magnetization enhancement in the vicinity of charge carriers in these microdomains probably accounts for the anomalous components in the Hall coefficient and the magnetization hysteresis observed in FeSi at LHe temperatures. The nature of the low-temperature transition at T[sub m]approx. 7 K in the system of interacting magnetic microparticles in iron monosilicide is discussed. © 2001 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We report the first results of microwave magnetoabsorption measurements (35-140GHz) in the intermediate-valence superconductor CeRu[sub 2]. The anomalous electrodynamic response is found to derive from a transition from a weak to a strong pinning regime in the superconducting mixed state of this unusual metal. The experimental results strongly support the appearance in the CeRu[sub 2] mixed state of a first-order phase transition that may be explained in terms of Fulde-Ferrel-Larkin-Ovchinnikov state formation. [ABSTRACT FROM AUTHOR]

The temperature dependence of the Hall coefficient in the interval 1.8–300 K is investigated in detail in high-quality single-crystal samples of a Kondo insulator — iron monosilicide. It is established that the parameter R[sub H](T,H=12.5 kOe) changes sign twice in the temperature interval employed, and at temperatures below T[sub m]approx. 7 K an anomalous (magnetic) component appears in the angular and field dependences of the Hall voltage. The results of the experimental investigations of R[sub H](T,H[sub 0]) in FeSi are discussed on the basis of the phase diagram in the model of an excitonic insulator. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The transport properties (Hall coefficient, thermopower, and resistivity) of high-quality singlecrystal samples of the classical mixed-valent compound SmB[sub 6] are investigated over a broad temperature range (1.6-300 K) in magnetic fields up to 45 T for the first time following the quasioptical measurements in the 0.6-4.5 meV frequency range [B. Gorshunov, N. Sluchanko, A. Volkov et al., submitted to Phys. Rev. B (1998)]. Measurements in the intrinsic conduction region permit determination of the gap width E[sub g] ≈ 20 meV and evaluation of the behavior of the mobility and concentration of light and heavy charge carriers, as well as the temperature dependence of the carrier relaxation time, in samarium hexaboride. The results of experimental investigations in the "impurity" conduction region (E[sub ex] ≈ 3.5 meV) are discussed within the Kikoin-Mishchenko exciton-polaron model of charge fluctuations. Arguments supporting the formation of a metallic state with an electron-hole liquid in SmB[sub 6] at liquid-helium temperatures are presented. [ABSTRACT FROM AUTHOR]

Thermopower of (BEDT-TTF)[sub m]X[sub n] organic conductors has been studied using a dedicated measurement technique in the temperature range of 4.2 to 300 K. It turned out that some features of the thermopower in quasi-two-dimensional metals, namely the presence of a peak in the thermopower of α-(BEDT-TTF)[sub 2]MHg(SCN)[sub 4] and a plateau in κ-(BEDT-TTF)[sub 2]Cu(NCS)[sub 2] in the temperature interval between 10 and 50 K, are probably due to the phonon drag effect. Similar temperature dependences of the Seebeck coefficient can be satisfactorily interpreted in terms of a simple model taking into account the real experimental curve of the phonon heat capacity versus temperature, C∝T[sup 2], which is not described by the Debye formula. One feature distinguishing organic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron–phonon scattering time τ[sub e–ph](T). Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)[sub 3]Cl[sub 2]·2H[sub 2]O organic conductor, which is characterized by a metal–insulator transition at T∼150 K. An analysis of measurements of the conductivity and thermopower vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T∼150 K) a metal–insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T∼20 K) a transition to a charge-density wave state takes place. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The thermopower coefficient as a function of temperature, S(T), has been measured in nonequilibrium superconductors, such as Al[sub 1-x]Si[sub x] substitutional solid solutions and Al–Si alloys on various decay stages. When aluminum is substituted with silicon, the contribution to the thermopower due to phonon-drag effects, which are dominant in pure aluminum at low temperatures, is suppressed, and low-temperature anomalies in S(T) detected in compositions near lattice instability limit are determined by the diffusion component of the thermopower. The low-temperature anomalies in the thermopower and the notable increase in the coefficient in front of the linear term in S(T) are attributed to effects of thermopower renormalization due to the electron–phonon interaction enhancement with “soft modes” in the face-centered cubic (FCC) lattice of Al[sub 1-x]Si[sub x] solid solutions. The nature of these anomalies in S(T) is analyzed in terms of the Kaiser and Reizer–Sergeev models. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The concentration of carriers in LuB12 is evaluated theoretically by applying ab initio FP-LMTO calculations. Theoretical results are found to be in agreement with high-precision measurements of the Hall coefficient RH(T) carried out on single crystals of the rare-earth dodecaborides RB12 (R=Ho,Er,Tm,Lu) at temperatures 1.8–300 K. The nature of the antiferromagnetic ordering in RB12 is investigated within an RKKY-like model supplemented by comprehensive electronic structure calculations for paramagnetic, ferromagnetic, and antiferromagnetic phases. [ABSTRACT FROM AUTHOR]

The galvanomagnetic and magnetic properties of EuB6 single crystal have been measured over wide temperature (1.8–300 K) and magnetic-field (up to 70 kOe) ranges, and the parameters of charge carriers and the characteristics of the magnetic subsystem are estimated in the paramagnetic and ferromagnetic ( T < T C ≈ 13.9 K) phases of this compound with strong electron correlations. In the temperature range T < T* ≈ 80 K, a magnetoresistance hysteresis Δρ( H)/ρ(0) is detected; it reaches a maximum amplitude of about 5% at T ≈ 12 K. The anomalies of charge transport observed in the temperature range T C < T < T* are shown to be related to the magnetic scattering of charge carriers ( m eff = (15–30) m 0, where m 0 is the free-electron mass) that results from a short-range magnetic order appearing upon the formation of ferromagnetic nanoregions (ferrons). [ABSTRACT FROM AUTHOR]

Detailed measurements of the Seebeck coefficient S( T) in a broad range of temperatures ( T = 2–300 K) have been performed for the first time for RB12 dodecaborides ( R = Ho, Er, Tm, Lu) in paramagnetic (diamagnetic for LuB12) and antiferromagnetic states. At intermediate temperatures (10–300 K), the thermopower is determined by the interaction of carriers with phonon modes, which are related to the oscillations of rare-earth atoms in the framework of atomic clusters B12. A comparative analysis of the parameters determining photon drag the thermopower related to the phonon drag and the results of galvanomagnetic measurements shows evidence for a significant effect of spin fluctuations on the behavior of charge transport characteristics in RB12 compounds with strong electron correlations. [ABSTRACT FROM AUTHOR]

Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related temperature Tf due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd0.73La0.27B6 and GdB6, which is caused by the random mutual shifts of Gd3+ spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law ΔH(T) ~ ((T − TD)/TD)α with α = − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd0.73La0.27B6 the resulting random spin configurations freeze at Tf ≈ 10.5 K where the maximum of magnetization is observed. Below Tf the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB6 the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at TN = 15.5 K, which prevails over the spin freezing at Tf ≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at T < 10 K. [ABSTRACT FROM AUTHOR]

Electron spin resonance (ESR) in strongly correlated metals is an exciting phenomenon, as strong spin fluctuations in this class of materials broaden extremely the absorption line below the detection limit. In this respect, ESR observation in CeB6 provides a unique chance to inspect Ce3+ magnetic state in the antiferroquadrupole (AFQ) phase. We apply the original high frequency (60 GHz) experimental technique to extract the temperature and angular dependences of g-factor, line width and oscillating magnetization. Experimental data show unambiguously that the modern ESR theory in the AFQ phase considering the Γ8 ground state of Ce3+ ion completely fails to predict both the g-factor magnitude and its angular dependence. Alignment of the external magnetic field along [100] axis induces a strong (more than twofold) broadening of ESR line width with respect to the other crystallographic directions and results also in the anomalous temperature dependences of the g-factor and oscillating magnetization. In this experimental geometry the latter parameter surprisingly exceeds total static magnetization by 20% at T* ~ 2.5 K. We argue that the unusual physical picture of ESR in CeB6 may be strongly affected by spin fluctuations and dynamic collective effects predominantly pronounced in [100] direction. [ABSTRACT FROM AUTHOR]

The antiferromagnetic ground state has been studied by transverse magnetoresistance, heat capacity and magnetization measurements, which were carried out on high quality single crystals of Tm0:996Yb0:004B12 dodecaboride in strong magnetic fields at liquid helium temperatures. Both antiferromagnetic-paramagnetic (AFP) and spin-orientation (AF1-AF2) phase transitions have been observed, and allowed to construct a complicated magnetic H-T phase diagram for this compound. Strong magnetoresistance anisotropy was found both in AF states (ρ(H∥[110])/ρ(H∥[111]) ~ 1:2 at H ~ 20 kOe) and at the critical field of AF-P transition (HN[100]/HN[111] ~ 1:25) in this magnetic metal with a simple fcc crystal structure. [ABSTRACT FROM AUTHOR]

The antiferromagnetic ground state has been studied by transverse magnetoresistance, heat capacity and magnetization measurements, which were carried out on high quality single crystals of Tm0.996Yb0.004B12 do-decaboride in strong magnetic fields at liquid helium temperatures. Both antiferromagnetic-paramagnetic (AF-P) and spin-orientation (AF1-AF2) phase transitions have been observed, and allowed to construct a complicated magnetic H-T phase diagram for this compound. Strong magnetoresistance anisotropy was found both in AF states (p(H∣∣[110])/p(H∣∣[111]) ~ 1:2 at H ~ 20 kOe) and at the critical field of AF-P transition (HN[100]/HN[111] ~ 1:25) in this magnetic metal with a simple fcc crystal structure. [ABSTRACT FROM AUTHOR]

Magnetic field dependences of the Hall coefficient RH and resistivity have been studied in detail for dilute magnetic compounds La1-xCexB6 (x ≤ 0.1) at temperatures 1.8-300 K. It was established that the regime of weak localization of charge carriers, which was observed in these heavy fermion systems below 30 K destroys gradually in magnetic field up to 8 T. Moreover, in addition to the strong negative magnetoresistance (Δ ρ/ρ ≈ 80%) a drastic enhancement of the negative Hall coefficient in magnetic field (Δ RH/RH ≈ 50%) has been deduced at liquid helium temperatures. The results of comprehensive analysis contradict the predictions of Kondo-impurity approach for this archetypal strongly correlated electron system in the dilute impurity limit. An alternative interpretation of La1-xCexB6 properties is developed on the basis of spin-polaron approach, Pauli paramagnetism and the density of states renormalization effects at low temperatures. [ABSTRACT FROM AUTHOR]